(1) Field of the Invention
The invention relates to systems and methods for measuring particle velocities in general and particularly to systems and methods that employ synthetic aperture methods to measure particle velocities.
(2) Description of the Prior Art
Efforts for resolving three-dimensional velocity fields are justified by the need to experimentally resolve flows that are highly three-dimensional and to validate numerical simulations of complex flows. The ability to spatio-temporally resolve flow features from small to large scales in arbitrarily large volumes is the goal of any three dimensional particle image velocimetry system. Of course, there have been many roadblocks to achieving all of these goals with a single system, and compromises must be made. Two-dimensional particle image velocimetry (2D PIV) is the most pervasive method for resolving velocity fields, thus it is not surprising that recent efforts to resolve three dimensional flow fields have extended many of the fundamentals of two dimensional particle image velocimetry to the third dimension.
Several methods exist for resolving three dimensional particle fields, or any three dimensional scenes for that matter, but the methods of data acquisition seem to fall into three broad categories: multiple-viewpoints, holography and internal optics alteration.
One of the earliest, but still frequently utilized, methods for three dimensional particle image velocimetry is two camera stereoscopic particle image velocimetry, which is primarily used to resolve the third component of velocity within a thin light sheet. A three dimensional particle tracking velocimetry (PTV) method is known which resolves the location of individual particles imaged by two, three or four cameras in a stereoscopic configuration. They report measurements in a large volume (e.g. 200×160×50 mm3), but with very low seeding density (≈1000 particles). Through precise calibration and knowledge of the imaging geometry, the particle field can be reconstructed. More recently, improvements to PTV methods have been made. In general, low seeding density is a typical limitation of PTV, yielding low spatial resolution in the vector fields.
Another technique which makes use of multiple viewpoints is defocusing digital particle image velocimetry (DDPIV). In theory, DDPIV capitalizes on the defocus blur of particles by placing an aperture with a defined pattern (usually pinholes arranged as an equilateral triangle) before the lens, which is a form of coded aperture imaging. The spread between three points generated by imaging a single particle corresponds to the distance from the camera along the Z dimension. In practice, the spread between particles is achieved using three off-axis pinhole cameras which causes a single point in space to appear at separate locations relative to the sensor of each camera. The images from all three camera sensors are superimposed onto a common coordinate system, an algorithm searches for patterns which form an equilateral triangle, and based on size and location of the triangle the three dimensional spatial coordinates of the point can be resolved. A main limitation of this technique appears to be seeding density, because the equilateral triangles formed by individual particles must be resolved to reconstruct the particle field. Simulations with seeding density of 0.038 particles per pixel (ppp) in a volume size of 100×100×100 mm3 have been reported, and experiments with seeding density of 0.034 ppp in a volume size of 150×150×150 mm3. The technique has also been efficiently implemented with a single camera using an aperture with color-coded pinholes, to measure velocity fields in a buoyancy driven flow in a 3.35×2.5×1.5 mm3 volume with seeding density≈0.001 ppp.
Tomographic-PIV also uses multiple viewpoints (usually 3-6 cameras) to obtain three dimensional velocity fields. Optical tomography reconstructs a three dimensional intensity field from the images on a finite number of cameras. The intensity fields are then subjected to three dimensional particle image velocimetry cross-correlation analysis. The seeding density for tomographic-PIV seems to be the largest attainable of the existing techniques. Simulations show volumetric reconstruction with seeding density of 0.05 ppp, and recent tomographic-PIV experiments typically have seeding density in the range of 0.02-0.08 ppp. The viewable depth of volumes in tomographic-PIV is typically three to five times smaller than the in-plane dimensions.
Holographic PIV (HPIV) is a technique in which the three-dimensional location of particles in a volume is deduced from the interference pattern of the light waves emanating from particles and the coherent reference wave that is incident upon the field. The nature of the interference pattern is used to back out information about the phase of light diffracted from objects in the volume, which is related to the distance of the objects from the sensor (i.e. depth in the volume). Holographic PIV makes use of this principle to image particle-laden volumes of fluids, and extract information about location of particles in the volume. In holography, the size of the observable volume is ultimately limited by the size and spatial resolution of the recording device. Very high resolution measurements of turbulent flow in a square duct using film-based HPIV have been reported, where particles were seeded to a reported density of 1-8 particles/mm3 in a volume measuring 46.6×46.6×42.25 mm3. Although film has much better resolution and is larger than digital recording sensors, difficulties of film based holographic PIV have been extensively cited, which have likely prevented the method from being widely utilized. In contrast, digital Holographic PIV is more readily usable, but is often limited to small volumes and low seeding density. A digital hybrid HPIV method has been implemented which allows for measurement in volumes with larger depth, but the size of the in-plane dimensions are limited by the physical size of the digital sensor, and seeding density remains low. Recent results of measurements in a turbulent boundary layer with increased seeding density (0.014 ppp) in a volume measuring 1.5×2.5×1.5 mm3 have been presented.
Also known in the prior art is Stroke, U.S. Pat. No. 3,785,262, which is said to disclose a method and apparatus for synthesizing large-aperture optics by exposure of a single photographic plate either successively or simultaneously through small-aperture optics. The technique represents the extension of the “synthetic-aperture radio telescope” principle to the optical domain by the relatively simple photographic synthesis of a “high-resolution” image in a single photograph, exposed either successively through sets of small “low-resolution” apertures successively placed to generate the spatial frequency components of the desired large aperture, or exposed simultaneously through a set of small “low-resolution” apertures having such optical characteristics and being so arranged as to generate the spatial frequency components of the desired large aperture.
Also known in the prior art is Kirk, U.S. Pat. No. 5,379,133, which is said to disclose an apparatus, a system, and a method wherein a synthetic aperture based sequence of image samples are generated with respect to a subject to be stereoscopically imaged. These sample images are presented to the spaced inputs of a holographic integrated combiner screen to be presented at an output aperture in laterally spaced mutual positioning. That spacing is selected, in one aspect, as one-half of the interpupillary distance of human eyes and thus binocular stereoscopic viewing at the aperture is achieved. The combiner screen may be utilized in conjunction with a holographic optical image combiner architecture which additionally employs a lens assembly such as a projecting lens to generate multi-zone outputs, each zone of which may be presented for stereoscopic viewing at a discrete viewing station. Correction for chromatic aberration of the holographic optical components is described.
Also known in the prior art is Adrian et al., U.S. Pat. No. 5,548,419, which is said to disclose a holographic particle image velocimeter that employs stereoscopic recording of particle images, taken from two different perspectives and at two distinct points in time for each perspective, on a single holographic film plate. The different perspectives are provided by two optical assemblies, each including a collecting lens, a prism and a focusing lens. Collimated laser energy is pulsed through a fluid stream, with elements carried in the stream scattering light, some of which is collected by each collecting lens. The respective focusing lenses are configured to form images of the scattered light near the holographic plate. The particle images stored on the plate are reconstructed using the same optical assemblies employed in recording, by transferring the film plate and optical assemblies as a single integral unit to a reconstruction site. At the reconstruction site, reconstruction beams, phase conjugates of the reference beams used in recording the image, are directed to the plate, then selectively through either one of the optical assemblies, to form an image reflecting the chosen perspective at the two points in time.
Also known in the prior art is Raffel et al., U.S. Pat. No. 5,610,703, which is said to disclose a digital particle image velocimetry (DPIV) method for contactless measurement of three dimensional flow velocities comprising the steps of seeding a flow with tracer particles; repeatedly illuminating a plane-like interrogation volume of the seeded flow; projecting the repeatedly illuminated interrogation volume onto at least a photo sensor in a projection direction for recording pictures of the illuminated interrogation volume; and determining the three dimensional flow velocities from the pictures of the repeatedly illuminated interrogation volume recorded by the photo sensor. The plane-like interrogation volume of the invention comprises at least two partial volumes positioned parallel to each other with regard to the projection direction. The step of repeatedly illuminating the interrogation volume comprises the step of illuminating the partial volumes in such a way that the pictures of different partial volumes are distinguishable from each other. The step of determining the three dimensional flow velocities of the flow comprises the steps of calculating a local autocorrelation function of a double exposed picture of the same partial volume, or calculating a local cross-correlation function between two separate pictures of the same partial volume, calculating a local cross-correlation function between two pictures of two different partial volumes, determining the sign of the out-of-plane component of the local flow velocities by using the location of a peak of the local cross-correlation function between the two pictures of the two different partial volumes, and determining the magnitude of the out-of-plane component of the local flow velocities by using the peak heights of peaks of both local correlation functions.
Also known in the prior art is McDowell et al., U.S. Pat. No. 5,905,568, which is said to disclose a system and a method for measuring three-dimensional velocities at a plurality of points in a fluid employing at least two cameras, positioned approximately perpendicular to one another. The cameras are calibrated to accurately represent image coordinates in world coordinate system. The two-dimensional views of the cameras are recorded for image processing and centroid coordinate determination. Any overlapping particle clusters are decomposed into constituent centroids. The tracer particles are tracked on a two-dimensional basis and then stereo matched to obtain three-dimensional locations of the particles as a function of time so that velocities can be measured therefrom. The stereo imaging velocimetry technique of the present invention provides a full-field, quantitative, three-dimensional map of any optically transparent fluid which is seeded with tracer particles.
Also known in the prior art is Meng et al., U.S. Pat. No. 6,496,262, which is said to disclose a holographic particle image velocimetry (HPIV) system that employs holograms of two time-separated particle fields, illuminated by separate reference beams on a single recording medium. 90-degree scattering is utilized for the object wave, in order to improve Numerical Aperture and resolve the third dimension of the hologram. The proposed HPIV system then uses substantially the same optical geometry for the reconstruction process. A CCD camera is utilized to extract particle subimages, thin slice by thin slice, and a centroid-finding algorithm is applied to extract centroid locations for each volume. The concise cross correlation (CCC) algorithm for extracting velocity vector fields from the centroid data is an important enabling feature of the proposed system. Correlations are calculated between subsets of centroids representing the images or cubes, and velocity vectors are computed from the individual correlations. Higher spatial resolution can also be obtained by pairing particle centroids individually.
Also known in the prior art is Schaller, U.S. Pat. No. 6,525,822, which is said to disclose a three-dimensional particle image velocimetry method, in which a stream system containing light-scattering particles is exposed continuously over a certain period or at least two discrete points in time using a laser light sheet, and a hologram is produced and evaluated. Increased accuracy in determining velocity is achieved by evaluating the hologram with regard to its phase information interferometrically by using a reconstruction wave and superimposing a reference wave.
Also known in the prior art is Japanese patent application publication JP2006058321A1, which is said to disclose a three-dimensional confocal microscopic system designed such that images used for micro PIV are acquired at the same focusing position by using accurate position information from an actuator. The three-dimensional confocal microscopic system includes: a confocal scanner for acquiring the slice images of a micro conduit as confocal images via a microscope; a video camera for outputting the image data of the confocal images; an actuator for moving the focusing position of the objective lens of the microscope in the direction of its optical axis; a control section for generating a scanning waveform signal for scanning the objective lens in the direction of the optical axis via the actuator; and an image processing section for calculating the speed of a fluid in the micro conduit on the basis of at least the two image data acquired by the video camera. Based upon a position signal output from the actuator, the system acquires at least the two images in a prescribed position in the micro conduit.
There is a need for improved particle image velocimetry in three dimensions.